Sintered ring magnet and method of manufacturing the same

ABSTRACT

A ring magnet manufacturing method includes the steps of stacking a plurality of radially oriented ring-shaped powder compacts ( 102 ) in an axial direction thereof to produce a ring-shaped powder compact rod, sintering the ring-shaped powder compact rod to produce a sintered ring-shaped powder compact rod ( 300 ) in which the ring-shaped powder compacts ( 102 ) are joined together, and dividing the sintered ring-shaped powder compact rod ( 300 ). In this ring magnet manufacturing method, protruding parts ( 103 ) are formed on upper end surfaces of the ring-shaped powder compacts ( 102 ) which will be located in uppermost layers of individual sintered ring magnets ( 100 ), for example, such that the ring-shaped powder compacts ( 102 ) are joined with a reduced joint strength at specific boundary regions of the sintered ring-shaped powder compact rod ( 300 ) where the protruding parts ( 103 ) are located than at the other boundary regions. The sintered ring magnets ( 100 ) are obtained by dividing the sintered ring-shaped powder compact rod ( 300 ) at the specific boundary regions having the reduced joint strength.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a radially oriented sintered ring magnet used in compact motors, for instance, and a method of manufacturing the radially oriented sintered ring magnet.

2. Description of the Background Art

A radially oriented anisotropic ring magnet is often used in permanent magnet motors. It is common practice to use an axially elongate ring magnet when manufacturing a compact high-power motor having a small inertia.

A problem encountered when pressing magnetic powder in a magnetic field for producing an axially elongate ring magnet is that a sufficient field strength of alignment field is not obtained, resulting in a reduction in alignment coefficient of the magnetic powder and an inability to achieve a high magnetic property.

When a ring magnet is radially oriented magnetically, a magnetic flux passing through a core of a metal die unit for pressing magnetic powder into a ring shape becomes equal to a magnetic flux passing inside a curved inner surface of a die. Therefore, expressing the inside diameter of the ring magnet (the core diameter of the metal die unit) as Di, the outside diameter of the ring magnet (the inside diameter of the die of the metal die unit) as Dd, the height of the ring magnet (the height of the die) as H, magnetic flux density within the core of the metal die unit as Bc, and magnetic flux density inside the curved inner surface of the die as Bd, there is a relationship given by equation (1) below: 2×p/4×Di ² ×Bc=p×Dd×H×Bd   (1)

A steel product, such as S45C, if used for making the core of the metal die unit, has a saturation flux density of approximately 1.5 T. Thus, substituting Bc=1.5 in equation (1) above and assuming that a magnetic field necessary for magnetic alignment is equal to or larger than 1.0 T which translates to Bd=1.0 T, the height H of the ring magnet which can be pressed with magnetic alignment is given by equation (2) below: H=3Di ²/4Dd   (2)

It is commonly known that a problem occurs due to deterioration of a property of magnetic alignment when pressing a ring magnet in a magnetic field if the ring magnet has an axial length larger than a value of H given by equation (2) above. In the context of this Specification, an “axially elongate ring magnet” refers to a ring magnet of which height (axial length) is larger than the value of H given by equation (2) above.

Accordingly, conventional practice has been to produce multiple ring magnet pieces, each having a short axial length which is within a range well allowing execution of pressing operation in a magnetic field, and join these ring magnet pieces with a bonding agent, for instance, to manufacture a ring magnet having a desired axial length.

Japanese Patent Application Publication No. 1990-281721 proposes a method of manufacturing a ring magnet having a desired axial length. According to the method of this Publication, an axially elongate ring magnet is produced in a step-by-step fashion by pressing magnetic powder in multiple layers as if by stacking powder compacts one on top of another in a metal die, each layer of the powder compacts having an axial length falling within a range which enables magnetic alignment.

Generally, a powder compact becomes reduced in size by 20% to 30% when sintered. A chronic problem encountered in the manufacture of a sintered ring magnet is that the powder compact does not shrink uniformly but becomes deformed as a result of shrinkage during sintering operation. This deformation of the powder compact occurs due to the influence of a difference in density within the powder compact, a difference in shrinkage ratio in a direction of magnetic alignment and other directions, and friction between the powder compact and a sintering tray, for example.

To overcome this deformation problem, Japanese Patent Application Publication No. 2001-335808 proposes a method of reducing deformation occurring in sintering operation by use of a compact restraining jig. According to the method of this Publication, a ring-shaped powder compact is placed to surround the compact restraining jig and sintered together with the restraining jig. The compact restraining jig serves to prevent deformation of the ring-shaped powder compact when the ring-shaped powder compact shrinks during the sintering operation.

The aforementioned method of Japanese Patent Application Publication No. 1990-281721 includes the steps of setting a specific metal die in a powder shaping press (pressing machine for pressing the magnetic powder in a magnetic field) incorporating an electromagnetic coil, filling magnetic powder into the metal die, forming each successive layer of a powder compact by pressing the magnetic powder in a magnetic field followed by demagnetizing operation, releasing the powder compact from the metal die, removing the powder compact from the pressing machine, and placing the powder compact on a vessel. Since a ring magnet is produced by making multiple layers of powder compacts by carrying out these sequential steps, the method of Japanese Patent Application Publication No. 1990-281721 leads to low manufacturing productivity. Also, since all these steps are performed in the pressing machine, there exist various limitations in operation in the individual steps. Furthermore, the powder compacts in lower layers are pressurized more times than those in upper layers, magnetic alignment is disturbed in the lower-layer powder compacts, inevitably causing deterioration of magnetic properties. Moreover, this ring magnet manufacturing method is associated with a problem that the magnetic properties deteriorate at each boundary region between the adjacent powder compacts due to the influence of the powder compact in a lower layer.

The aforementioned method of Japanese Patent Application Publication No. 2001-335808 has a problem that the compact restraining jig is a consumable and it is necessary to set the compact restraining jig with a central axis thereof precisely aligned with a central axis of the ring-shaped powder compact, resulting in low manufacturing productivity. Furthermore, as the ring-shaped powder compact is sintered on a sintering jig (setter), a lower part of the ring-shaped powder compact does not normally shrink in the sintering operation due to friction with the setter. For this reason, greater deformation is likely to occur in the lower part of the ring-shaped powder compact.

SUMMARY OF THE INVENTION

In light of the aforementioned problems of the prior art, it is an object of the invention to provide a sintered ring magnet featuring a high degree of magnet shape accuracy and a method of manufacturing such a ring magnet with high productivity.

In one principal aspect of the invention, a method of manufacturing sintered ring magnets includes the steps of stacking a plurality of radially oriented ring-shaped powder compacts in an axial direction thereof to produce a ring-shaped powder compact rod, sintering the ring-shaped powder compact rod to produce a sintered ring-shaped powder compact rod in which the ring-shaped powder compacts are joined together as a result of sintering operation, and dividing the sintered ring-shaped powder compact rod. In this ring magnet manufacturing method, the ring-shaped powder compacts are joined to one another with a reduced joint strength at specific boundary regions than at the other boundary regions when sintered, and the sintered ring magnets are obtained by dividing the sintered ring-shaped powder compact rod at the specific boundary regions having the reduced joint strength.

According to this ring magnet manufacturing method of the invention, several ring magnets joined into a single structure, or the sintered ring-shaped powder compact rod, by the sintering operation are handled together in subsequent processes. This approach of the invention produces the following advantageous effects. Specifically, since several sintered ring magnets are handled as a single structure, the sintered ring magnets can be transferred together from one process to next with high efficiency. Since curved inner surfaces and outer surfaces of several sintered ring magnets can be machined together in the form of the sintered ring-shaped powder compact rod, it is possible to achieve improved machining efficiency. Since several sintered ring magnets can be subjected together to an anticorrosion surface treatment in the form of the sintered ring-shaped powder compact rod, it is possible to achieve improved treatment efficiency. Additionally, it is possible to produce the sintered ring-shaped powder compact rod with a high degree of shape accuracy.

These and other objects, features and advantages of the invention will become more apparent upon reading the following detailed description along with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a sintered ring magnet according to a first embodiment of the invention;

FIG. 2 is a cross-sectional view of a sintered ring-shaped powder compact rod from which a plurality of sintered ring magnets of the first embodiment are produced;

FIG. 3 is a cross-sectional view of the sintered ring magnets obtained by dividing the sintered ring-shaped powder compact rod of FIG. 2;

FIG. 4 is a cross-sectional view of one of the sintered ring magnets of the first embodiment;

FIG. 5 is a cross-sectional view of a rotor formed by mounting the sintered ring magnet of the first embodiment on a rotor shaft;

FIG. 6 is a plan view generally showing the configuration of a manufacturing system for pressing magnetic powder to form ring-shaped powder compacts used for producing the sintered ring magnet of the first embodiment;

FIGS. 7A and 7B are a plan view and a cross-sectional view, respectively, showing the structure of a transferable metal die unit;

FIGS. 8A, 8B and 8C are cross-sectional views showing different types of upper punches according to the first embodiment of the invention;

FIGS. 9A, 9B and 9C are cross-sectional views showing the structure and working of a powder feeding unit;

FIGS. 10A, 10B, 10C and 10D are cross-sectional views showing the structure and working of a punch setup unit;

FIGS. 11A, 11B and 11C are cross-sectional views showing the structure and working of a pressing machine;

FIGS. 12A and 12B are cross-sectional views showing the structure of a pressing device;

FIGS. 13A and 13B are plan views and FIGS. 13C and 13D are cross-sectional views taken along lines A-A and B-B of FIGS. 13A and 13B, respectively, showing the structure of a back core;

FIG. 14 is a cross-sectional view showing a state of magnetic fluxes generated during radial magnetic alignment operation;

FIGS. 15A and 15B are a plan view and a cross-sectional view, respectively, showing the structure of a die release unit, wherein FIG. 15A is the plan view as seen in a direction of arrows A-A of FIG. 15B;

FIGS. 16A, 16B, 16C and 16D are cross-sectional views showing the working of the die release unit;

FIGS. 17A and 17B are cross-sectional views showing the structure and working of a powder removal unit for removing excess magnetic powder adhering to each powder compact;

FIGS. 18A and 18B are cross-sectional views showing operation performed at the powder removal unit of FIGS. 17A and 17B;

FIGS. 19A and 19B are cross-sectional views showing the structure and working of a stacking unit;

FIGS. 20A and 20B are cross-sectional views also showing the structure and working of the stacking unit;

FIGS. 21A and 21B are cross-sectional views also showing the structure and working of the stacking unit;

FIG. 22 is a cross-sectional view of a sintered ring-shaped powder compact rod from which a plurality of sintered ring magnets according to a second embodiment of the invention are produced;

FIG. 23 is a cross-sectional view of the sintered ring magnets obtained by dividing the sintered ring-shaped powder compact rod of FIG. 22;

FIG. 24 is a cross-sectional view of a sintered ring-shaped powder compact rod from which a plurality of sintered ring magnets according to a third embodiment of the invention are produced; and

FIG. 25 is a cross-sectional view of a sintered ring-shaped powder compact rod from which a plurality of sintered ring magnets according to a fourth embodiment of the invention are produced.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Specific embodiments of the present invention are now described in detail with reference to the accompanying drawings.

First Embodiment

(1) Structure of the Sintered Ring Magnet of the First Embodiment

First, the structure of a sintered ring magnet 100 according to a first embodiment of the invention is described with reference to FIG. 1 which is a perspective view of the sintered ring magnet 100.

Referring to FIG. 1, the sintered ring magnet 100 of the embodiment is a ring magnet produced by stacking a plurality of ring-shaped powder compacts 102 in layers (three layers in the illustrated example) in an axial direction and joining the ring-shaped powder compacts 102 by sintering the stacked ring-shaped powder compacts 102. The individual ring-shaped powder compacts 102 are joined at boundaries 101 as a result of sintering operation, together forming a single structure. As depicted in FIG. 1, there is formed a ring-shaped protruding part 103 on an upper end surface of the ring-shaped powder compact 102 in an uppermost layer of the sintered ring magnet 100 in this embodiment. Although not illustrated, side surfaces of the protruding part 103 are preferably tapered upward to form a narrower upper end.

(2) Method of Manufacturing the Sintered Ring Magnet of the First Embodiment

Next, a method of manufacturing the sintered ring magnet 100 of the first embodiment is described.

Generally, in the manufacture of a radially oriented ring magnet, there are limitations on the axial length of a ring-shaped powder compact which can be magnetically oriented by applying an alignment magnetic field in a single operation. Therefore, common practice for manufacturing a radially oriented ring magnet having a large axial length is to produce a plurality of ring-shaped powder compacts by pressing magnetic powder in a metal die, each of the ring-shaped powder compacts having an axial length which can be radially oriented in a magnetic field, stack the multiple ring-shaped powder compacts removed from the metal die in an axial direction, and join the ring-shaped powder compacts to form a single structure by performing a sintering process.

According to the method of manufacturing the sintered ring magnet 100 of the first embodiment, a larger number of ring-shaped powder compacts 102 than producing a single sintered ring magnet 100 are stacked as shown in FIG. 2. As can be seen from FIG. 2, specific ones of these ring-shaped powder compacts 102 have the aforementioned protruding parts 103 formed on the upper end surfaces. The stacked ring-shaped powder compacts 102 as shown in FIG. 2 are sintered and subjected to a subsequent heat treatment. Consequently, the ring-shaped powder compacts 102 are joined at boundaries thereof, together forming a sintered ring-shaped powder compact rod 300. While all of the ring-shaped powder compacts 102 are joined together at the boundaries, the ring-shaped powder compacts 102 having the protruding parts 103 each have a smaller contact area with the ring-shaped powder compacts 102 located immediately above. Therefore, joint strength at the boundaries where the protruding parts 103 are provided is smaller than at the other boundaries.

A curved inner surface and a curved outer surface of the sintered ring-shaped powder compact rod 300, in which the ring-shaped powder compacts 102 are joined into a single structure by sintering, are finished by machining, and the sintered ring-shaped powder compact rod 300 is subjected to an anticorrosion surface treatment. Then, a mechanical bending stress is applied to boundary regions of the sintered ring-shaped powder compact rod 300 and, as a result, the sintered ring-shaped powder compact rod 300 breaks at the boundary regions where the protruding parts 103 are formed and multiple sintered ring magnets 100 are obtained as shown in FIG. 3. Even without application of an external mechanical force, it would be possible to divide the sintered ring-shaped powder compact rod 300 at the boundary regions where the protruding parts 103 are located by simply applying a pulsed magnetic field during magnetizing operation, as a result of an impact produced during magnetization.

Instead of finishing the curved inner surface of the sintered ring-shaped powder compact rod 300 by machining operation as stated above, curved inner surfaces of the individual sintered ring magnets 100 obtained by dividing the sintered ring-shaped powder compact rod 300 at the boundary regions may be machined according to the invention. Also, because the sintered ring-shaped powder compact rod 300, and thus the sintered ring magnets 100 obtained by dividing the sintered ring-shaped powder compact rod 300, have a high degree of shape accuracy, each of the sintered ring magnets 100 may be fitted on and bonded to a rotor shaft without performing any finishing operation on the curved inner surface.

FIG. 4 is a cross-sectional view of one of the sintered ring magnets 100 obtained by dividing the sintered ring-shaped powder compact rod 300, and FIG. 5 is a cross-sectional view of a rotor formed by mounting the sintered ring magnet 100 of FIG. 4 on a rotor shaft 200. Referring to the cross section of FIG. 4, thick lines 105 indicate surface areas of the sintered ring magnet 100 where the surface treatment is performed. The surface treatment is not performed on areas of the sintered ring magnet 100 shown by thin lines in the cross section of FIG. 4. These untreated surface areas of the sintered ring magnet 100 correspond to the boundary regions where the sintered ring-shaped powder compact rod 300 has been divided at the protruding parts 103 and, more particularly, an upper end surface of the protruding part 103 and a portion of a lower end surface of the sintered ring magnet 100 where the protruding part 103 of the sintered ring magnet 100 immediately below has been joined. The sintered ring magnet 100 will not corrode from these untreated surface areas, however, because the untreated surface areas of the sintered ring magnet 100 are fully covered with a bonding agent 210 in a process of bonding the sintered ring magnet 100 to the rotor shaft 200.

Generally, when a powder compact is sintered, the powder compact is placed on a tray in which parting powder is dusted and input into a sintering furnace. The parting powder is used for preventing the powder compact from sticking to the tray as a result of sintering operation. The parting powder also serves to decrease friction between the powder compact (sintered powder compact) and the tray when sintering shrinkage occurs during the sintering operation to thereby prevent sintering deformation. When a ring-shaped powder compact is sintered, a disturbance to normal sintering shrinkage due to friction between the powder compact and the tray can not be avoided by use of the parting powder alone, and this results in the occurrence of sintering deformation. Typically, a lower part of the ring-shaped powder compact which is in contact with the tray deforms into an oval shape or increases in size.

Because the sintered ring-shaped powder compact rod 300 of the present embodiment is formed by stacking a plurality of ring-shaped powder compacts 102 in the axial direction and sintering the stacked ring-shaped powder compacts 102, deformation due to the sintering shrinkage occurs only in the ring-shaped powder compact 102 in a lowermost layer and the ring-shaped powder compacts 102 in upper layers maintain their ring shape with high shape accuracy even after the sintering operation. Therefore, it is preferable that the ring-shaped powder compact 102 to be used in the lowermost layer of the sintered ring-shaped powder compact rod 300 be shaped to have a smaller inside diameter so that the curved inner surface of that ring-shaped powder compact 102 can be finished with a sufficient machining allowance even if the sintering deformation occurs. Although the ring-shaped powder compact 102 in only the lowermost layer must have greater machining allowance, the sintered ring-shaped powder compact rod 300 thus manufactured has a high degree of shape accuracy as a whole.

According to the above-described ring magnet manufacturing method, several ring magnets 100 joined into a single structure, or the sintered ring-shaped powder compact rod 300, in the sintering process are handled together in subsequent processes. This approach of the invention produces the following advantageous effects:

(a) Since several sintered ring magnets 100 are handled as a single structure, the sintered ring magnets 100 can be transferred together from one process to next with high efficiency.

(b) Since the curved inner surfaces and outer surfaces of several sintered ring magnets 100 can be machined together in the form of the sintered ring-shaped powder compact rod 300, it is possible to achieve improved machining efficiency.

(c) Since several sintered ring magnets 100 can be subjected together to the anticorrosion surface treatment in the form of the sintered ring-shaped powder compact rod 300, it is possible to achieve improved treatment efficiency.

(d) It is possible to produce the sintered ring-shaped powder compact rod 300 with a high degree of shape accuracy.

According to the ring magnet manufacturing method of the present embodiment, the protruding parts 103 formed on the upper end surfaces of the specific ring-shaped powder compacts 102 need not necessarily be ring-shaped as illustrated in FIG. 1, but protruding parts of any shape which reduce contact areas with the ring-shaped powder compacts 102 immediately above may be formed on the upper end surfaces of the specific ring-shaped powder compacts 102. For example, a ring-shaped protruding part having a generally semicircular cross-sectional shape or multiple protruding parts separated along a circumferential direction may be formed on the upper end surface of each specific ring-shaped powder compact 102. Also, instead of the protruding parts, recessed parts may be formed in the upper end surfaces of the specific ring-shaped powder compacts 102 for reducing contact areas with the adjacent ring-shaped powder compacts 102. Furthermore, such protruding parts or recessed parts need not necessarily be formed on the upper end surfaces of the specific ring-shaped powder compacts 102 but may be formed on lower end surfaces of the ring-shaped powder compacts 102 which will be located at the bottom of the individual sintered ring magnets 100. Moreover, the protruding parts or the recessed parts may be formed on both the upper end surfaces of the ring-shaped powder compacts 102 which will be located at the top of the individual sintered ring magnets 100 and the lower end surfaces of the ring-shaped powder compacts 102 which will be located at the bottom of the individual sintered ring magnets 100.

(3) Procedure for Manufacturing the Sintered Ring Magnet of the First Embodiment

Now, a procedure for manufacturing the sintered ring magnet 100 of the first embodiment is specifically described.

The sintered ring magnet 100 is a permanent magnet manufactured by using as a raw material neodymium magnetic alloy containing neodymium (Nd), dysprosium (Dy), iron (Fe) and boron (B). The neodymium magnetic alloy is subjected to a hydrogen occlusion treatment and finely pulverized by use of a jet mill to obtain fine alloy powder (magnetic powder) having an average particle size of about 5 micrometers. The ring-shaped powder compacts 102 stacked in multiple layers of each sintered ring magnet 100 are made by pressing this powder.

FIG. 6 is a plan view generally showing the configuration of a manufacturing system for pressing the magnetic powder to form the ring-shaped powder compacts 102 used for producing the sintered ring magnets 100 of the first embodiment. As illustrated in FIG. 6, the manufacturing system includes a belt conveyor 2 for transferring a transferable metal die unit 10, a powder feeding unit 3 for weighing the magnetic powder and filling the same into a ring-shaped cavity 10 h formed in the transferable metal die unit 10, a punch setup unit 4 for setting up an upper punch log for pressurizing the magnetic powder in a condition where the upper punch log can press the magnetic powder filled in the cavity 10 h of the transferable metal die unit 10, a pressing unit 5 for pressing the magnetic powder in a magnetic field in the transferable metal die unit 10 in which the upper punch log has been set in position for readily performing pressing operation, a die release unit 6 for drawing a ring-shaped powder compact which has been pressed in the magnetic field out of the transferable metal die unit 10, a powder removal unit 7 for removing excess magnetic powder adhering to each ring-shaped powder compact which has been drawn out of the transferable metal die unit 10, a stacking unit 8 for stacking ring-shaped powder compacts which have been pressed in the magnetic field, and a powder removal/die setup unit 9 for removing the magnetic powder adhering to the transferable metal die unit 10 and setting the transferable metal die unit 10 in a transferable state.

As shown in FIGS. 7A and 7B, the transferable metal die unit 10 includes a palette 10 a which travels on the belt conveyor 2, a first holder 10 b for holding a lower metal die portion, a columnlike core 10 d, a lower punch 10 e, a die 10 f which forms the aforementioned cavity 10 h into which the magnetic powder is filled together with the core 10 d and the lower punch 10 e, the core 10 d being disposed inside the die 10 f on a common axis therewith, and the aforementioned upper punch log held on a second holder 10 j.

FIGS. 8A, 8B and 8C are cross-sectional views showing upper punches log having different shapes. As will be later discussed, the upper punch log serves to compress the magnetic powder filled in the cavity 10 h from top side. In the transferable metal die unit 10 of the present embodiment, there is a choice of three different types of upper punches log. These are the upper punch log shown in FIG. 8A having a flat bottom end, the upper punch log shown in FIG. 8B having a groove g1 formed in the bottom end, and the upper punch 10 g shown in FIG. 8C having a tapered groove g2 formed in the bottom end. The upper punch 10 g shown in FIG. 8A is for producing a ring-shaped powder compact 102 having a flat upper end surface, the upper punch log shown in FIG. 8B is for producing a ring-shaped powder compact 102 with a protruding part 103 having a rectangular cross section formed on the upper end surface of the ring-shaped powder compact 102, and the upper punch log shown in FIG. 8C is for producing a ring-shaped powder compact 102 with a protruding part 103 having an upward-narrowing trapezoidal cross section formed on the upper end surface of the ring-shaped powder compact 102.

First, the transferable metal die unit 10 is transported to the powder feeding unit 3 by the belt conveyor 2. FIGS. 9A, 9B and 9C are cross-sectional views showing the structure and working of the powder feeding unit 3.

Shown in FIG. 9A is a powder weighing process in which the powder feeding unit 3 feeds a particular weight of magnetic powder 11, such as the aforementioned neodymium magnetic alloy, into a vessel 3 c by use of a vibration feeder while measuring the magnetic powder 11 with a weighing instrument.

Shown in FIGS. 9B and 9C is a powder feeding process in which a powder feeding jig 3 a having a funnellike structure for guiding the magnetic powder 11 into the cavity 10 h of the transferable metal die unit 10 and a winglike jig (not shown) for stirring the magnetic powder 11 fed into the cavity 10 h are set up on the die 10 f of the transferable metal die unit 10 and, then, the vessel 3 c accommodating the magnetic powder 11 is moved up the position of the funnellike powder feeding jig 3 a and turned to a slant angle so that the magnetic powder 11 in the vessel 3 c is transferred into the powder feeding jig 3 a. Then, the powder feeding jig 3 a is vibrated by a vibrating mechanism 3 b to fully transfer the magnetic powder 11 on the powder feeding jig 3 a into the cavity 10 h, and the aforementioned winglike jig is turned so that blades of the winglike jig rotate while gradually ascending to stir up the magnetic powder 11 filled in the cavity 10 h. As the magnetic powder 11 filled in the cavity 10 h is thoroughly mixed by the rotating blades of the winglike jig, voids in the magnetic powder 11 or bridging thereof, if any present in the cavity 10 h, are caused to collapse so that the cavity 10 h of the transferable metal die unit 10 is uniformly charged with the magnetic powder 11.

The transferable metal die unit 10 of which cavity 10 h has been charged with the magnetic powder 11 is then transported to the punch setup unit 4 by the belt conveyor 2. FIGS. 10A, 10B, 10C and 10D are cross-sectional views showing the structure and working of the punch setup unit 4.

As shown in these Figures, the punch setup unit 4 includes a tong-grip lifter 4 a for catching the upper punch 10 g and a transfer mechanism (not shown) for raising and lowering the tong-grip lifter 4 a and moving the upper punch log caught by the tong-grip lifter 4 a. With this punch setup unit 4, it is possible to set the transferable metal die unit 10 in a condition where the magnetic powder 11 in the cavity 10 h can be pressurized by the upper punch log.

First, the palette 10 a loaded with the transferable metal die unit 10 is transported onto a stage of the punch setup unit 4 and set at a prescribed position as shown in FIG. 10A. Then, the tong-grip lifter 4 a descends and catches the upper punch 10 g as shown in FIG. 10B. The tong-grip lifter 4 a lifts the upper punch 10 g as shown in FIG. 10C, moves toward a lower die section and descends to fit the upper punch 10 g over the core 10 d as shown in FIG. 10D. Subsequently, the tong-grip lifter 4 a releases the upper punch log and the upper punch 10 g fits in the cavity 10 h. The diameter of an upper end portion of the core 10 d is made smaller than the diameter of a lower portion of the core 10 d located in the cavity 10 h by 0.2 mm and tapered by 3°. Therefore, even if the position of the palette 10 a deviates from that of the tong-grip lifter 4 a by an amount not exceeding 0.1 mm at the time of punch insertion, there occurs no such a failure that the upper punch 10 g can not be fitted over the core 10 d. After releasing the upper punch 10 g, the tong-grip lifter 4 a ascends and moves back to its original position.

The transferable metal die unit 10 fitted with the upper punch 10 g is advanced to a pressing stage at the pressing unit 5 by the belt conveyor 2. FIGS. 11A, 11B and 11C are cross-sectional views showing the structure and working of the pressing unit 5, FIGS. 12A and 12B are cross-sectional views showing the structure of a pressing device 5 c, and FIGS. 13A and 13B are plan views and FIGS. 13C and 13D are cross-sectional views taken along lines A-A and B-B of FIGS. 13A and 13B, respectively, showing the structure of a back yoke 5 d.

As shown in FIG. 6, the pressing stage is provided with a transfer mechanism 5 h for transferring the transferable metal die unit 10 fitted with the upper punch 10 g from the palette 10 a on the belt conveyor 2 to the pressing unit 5 and returning the transferable metal die unit 10 back onto the palette 10 a on the belt conveyor 2 upon completion of pressing operation. Referring to FIGS. 11A, 11B and 11C, the pressing unit 5 includes upper and lower electromagnetic coils 5 a (fixed to upper and lower frames) for generating an alignment magnetic field for magnetically aligning the magnetic powder 11, a pressing mechanism 5 b for raising and lowering the aforementioned pressing device 5 c for forcing the upper electromagnetic coil 5 a and the upper punch 10 g down, an up/down drive mechanism for raising and lowering the upper frame including the upper electromagnetic coil 5 a and the pressing mechanism 5 b, a ring-shaped elastic member 5 j, and the aforementioned back yoke 5 d which goes into contact with the die 10 f when actuated by an unillustrated air cylinder.

Referring to FIGS. 12A and 12B, the pressing device 5 c includes a punch pressing portion 5 e for pressing the upper punch 10 g, a movable rod 5 f which moves as if pushed into the interior of the punch pressing portion 5 e, and a spring 5 g located between a rear surface of the movable rod 5 f and an inner surface of the punch pressing portion 5 e for pressing the movable rod 5 f against the core 10 d.

Referring to FIGS. 13A, 13B, 13C and 13D, the back yoke 5 d is made of a pair of ferromagnetic members each having a semicircular inner recess which fits on a curved outer surface of the die 10 f from both sides. The back yoke 5 d is first placed in a manner that a central axis of the back yoke 5 d aligns with that of the die 10 f and moved toward the die 10 f from both sides until the semicircular inner recesses of the ferromagnetic members of the back yoke 5 d go into contact with the die 10 f.

When the transferable metal die unit 10 has been advanced from the punch setup unit 4 to the pressing unit 5 by the belt conveyor 2, a metal die portion of the transferable metal die unit 10 is transferred from the palette 10 a to a pressing portion of the pressing unit 5 together with the first holder 10 b by the transfer mechanism 5 h as shown in FIG. 11A.

Next, as the up/down drive mechanism is actuated, the electromagnetic coil 5 a and the pressing device 5 c descend, the upper and lower frames are fixed to each other by a clamping function thereof and the die 10 f is secured in position by the ring-shaped elastic member 5 j which is attached to a bottom part of the upper frame as shown in FIG. 11B. Then, the ferromagnetic members of the back yoke 5 d approach the die 10 f from both sides thereof and go into tight contact with the curved outer surface of the die 10 f. Subsequently, currents are flowed through the upper and lower electromagnetic coils 5 a to generate a radially aligning magnetic field, and the pressing device 5 c is caused to descend to force the upper punch log downward as shown in FIG. 11C. Consequently, the upper punch log presses the magnetic powder 11 filled in the cavity 10 h, whereby a radially oriented ring-shaped powder compact 102 is obtained. Compacting pressure should be 10 to 100 MPa, preferably 40 MPa, and the intensity of the aligning magnetic field should be made equal to or higher than 1 T.

FIG. 14 is a cross-sectional view showing a state of magnetic fluxes generated during radial magnetic alignment operation. A magnetic field generated by the upper electromagnetic coil 5 a passes through the pressing device 5 c which is a ferromagnetic member in the form of a magnetic flux and enters the movable rod 5 f which is also a ferromagnetic member, whereas a magnetic field generated by the lower electromagnetic coil 5 a passes through the first holder 10 b which is a ferromagnetic member and enters the core 10 d. The lower punch 10 e and the upper punch 10 g are nonmagnetic members.

As shown in FIG. 14, the magnetic fluxes indicated by arrows with broken lines pass through the movable rod 5 f and the core 10 d which are ferromagnetic members and pass through the cavity 10 h within the die 10 f which is a ferromagnetic member in radial directions thereof, creating thereby a radially aligning magnetic field inside the cavity 10 h.

The radially oriented ring-shaped powder compact 102 is returned onto the palette 10 a together with the transferable metal die unit 10 by the transfer mechanism 5 h.

In the above-described pressing stage, the intensity of the aligning magnetic field applied in the pressing operation can be controlled by regulating the amount of current to be flowed through each electromagnetic coil 5 a. It is possible to adjust the alignment coefficient of the ring-shaped powder compact 102 to be used in each layer by regulating the current to be flowed through each electromagnetic coil 5 a. This makes it possible to ring-shaped powder compacts to be used in individual layers that have specific magnetic properties described with reference to the first to third embodiments.

Next, the transferable metal die unit 10 carrying the ring-shaped powder compact 102 is advanced to a die release stage at the die release unit 6 by the belt conveyor 2. FIGS. 15A and 15B are a plan view and a cross-sectional view, respectively, showing the structure of the die release unit 6, wherein FIG. 15A is the plan view as seen in a direction of arrows A-A of FIG. 15B.

As shown in these Figures, the die release unit 6 is provided with a pressing mechanism including an air cylinder 6 a for pressurizing the ring-shaped powder compact 102 and an upper punch stopper 6 d, and a die lifting mechanism including a table 6 c upward and air cylinders 6 b for lifting the die 10 f upward.

FIGS. 16A, 16B, 16C and 16D are cross-sectional views showing a die release process performed at the die release unit 6. The palette 10 a loaded with the transferable metal die unit 10 carrying the ring-shaped powder compact 102 is transported by the belt conveyor 2 and set at a prescribed position on the die release stage. As the air cylinder 6 a lifts up the palette 10 a as shown in FIG. 16A, the upper punch 10 g goes in contact with the upper punch stopper 6 d so that the ring-shaped powder compact 102 is pressurized. Pressure applied by the air cylinder 6 a to the ring-shaped powder compact 102 should be 0.1 to 1 MPa.

Next, as shown in FIG. 16B, the air cylinders 6 b are actuated and the table 6 c forced upward by the air cylinders 6 b lifts up the die 10 f, whereby the ring-shaped powder compact 102 is released from the die 10 f.

Subsequently, the air cylinder 6 a retracts and the palette 10 a lies on the belt conveyor 2 as shown in FIG. 16C. Then, the belt conveyor 2 advances the palette 10 a up to a position where the die 10 f will be loaded on the second holder 10 j placed on the palette 10 a when the die 10 f supported by the table 6 c descends as shown in FIG. 16D. When the palette 10 a reaches this position, the air cylinders 6 b retract so that the table 6 c descends, thereby placing the die 10 f on the second holder 10 j.

In the die release process for drawing out the ring-shaped powder compact 102 from the transferable metal die unit 10, there is a difference in internal stress between an upper portion of the ring-shaped powder compact 102 drawn out of the transferable metal die unit 10 and a lower portion of the ring-shaped powder compact 102 still remaining in the transferable metal die unit 10. Generally, cracks are likely to develop in a boundary region between the upper portion of the ring-shaped powder compact 102 drawn out of the transferable metal die unit 10 and the lower portion of the ring-shaped powder compact 102 remaining in the transferable metal die unit 10 due to the difference in internal stress. In the die release unit 6 of this embodiment, however, the ring-shaped powder compact 102 is drawn out of the die 10 f under conditions where the ring-shaped powder compact 102 is pressurized and, therefore, the difference in internal stress between the upper and lower portions of the ring-shaped powder compact 102 is so small that the occurrence of cracks is avoided.

The transferable metal die unit 10 from which the ring-shaped powder compact 102 has been drawn out is advanced to the powder removal unit 7 by the belt conveyor 2. FIGS. 17A, 17B, 18A and 18B are cross-sectional views showing the structure and working of the powder removal unit 7.

As shown in these Figures, the powder removal unit 7 for performing a powder removal process is provided with a raise/lower mechanism including a table 7 a and air cylinders 7 b for raising and lowering the table 7 a, a nozzle 7 c for spewing out nitrogen gas and a dust collecting duct 7 d for drawing and collecting excess magnetic powder and iron powder into a dust collector. As the excess magnetic powder adhering to the ring-shaped powder compact 102 is removed at the powder removal unit 7, it is possible to prevent the ring-shaped powder compact 102 from listing or shifting away from a normal position in a stacking process at a succeeding stage.

The palette 10 a loaded with the transferable metal die unit 10 from which the ring-shaped powder compact 102 has been drawn out is transported by the belt conveyor 2 and set at a prescribed position in the powder removal unit 7, and then the air cylinders 7 b are actuated, causing the table 7 a to ascend as shown in FIG. 17A. Consequently, as shown in FIG. 17B, the lower punch 10 e supported by the table 7 a ascends, whereby the ring-shaped powder compact 102 is removed from the core 10 d. When the ring-shaped powder compact 102 is drawn out from the core 10 d, the ring-shaped powder compact 102 scrapes off excess magnetic powder adhering to the a curved outer surface of the core 10 d, so that the excess magnetic powder remains adhering to and around upper and lower ends of a curved inner surface of the ring-shaped powder compact 102. When the ring-shaped powder compact 102 is removed from the core 10 d, the upper punch 10 g is also removed and placed on the second holder 10 j.

When the upper end surface of the ring-shaped powder compact 102 has slightly protruded above the core 10 d in the aforementioned process of drawing out the ring-shaped powder compact 102 from the core 10 d, the upper punch 10 g is removed, nitrogen gas is spewed out from the nozzle 7 c to blow off the magnetic powder adhering to surfaces of the ring-shaped powder compact 102, and the magnetic powder is sucked up by the dust collecting duct 7 d as shown in FIG. 18A. Subsequently, the ring-shaped powder compact 102 is drawn further upward as shown in FIG. 18B. It is to be noted however that the ring-shaped powder compact 102 need not necessarily be drawn out completely from the core 10 d.

FIGS. 19A, 19B, 20A, 20B, 21A and 21B are cross-sectional views showing the structure and working of the stacking unit 8. As shown in these Figures, the stacking unit 8 is provided with a gripping mechanism including a tong-grip lifter 8 a for catching the ring-shaped powder compact 102, a table 8 b on which multiple ring-shaped powder compacts 102 are stacked, a mechanism (not shown) for positioning, raising, lowering and moving the tong-grip lifter 8 a, and a turning mechanism (not shown) for turning the table 8 b. In one form of the embodiment, an electromagnetic gripping device may be used as the aforementioned gripping mechanism.

First, the tong-grip lifter 8 a of the gripping mechanism is moved to a position just above the ring-shaped powder compact 102 drawn from the core 10 d as shown in FIG. 19A. Then, the tong-grip lifter 8 a is lowered to grip the ring-shaped powder compact 102 as shown in FIG. 19B. Gripping force exerted by the tong-grip lifter 8 a is adjusted to 0.1 to 4N. Next, the tong-grip lifter 8 a is raised and the tong-grip lifter 8 a is moved such that the center of the tong-grip lifter 8 a is located exactly above the table 8 b as shown in FIG. 20A. Then, the tong-grip lifter 8 a is lowered and the ring-shaped powder compact 102 is placed on the table 8 b as shown in FIG. 20B. Further, using the same procedure as explained above, ring-shaped powder compacts 102 for second and third layers are stacked on top of the ring-shaped powder compact 102 in a first layer (bottom layer) as shown in FIGS. 21A and 21B. A desired number of ring-shaped powder compacts 102 may be stacked by repeating the aforementioned stacking procedure.

To obtain the sintered ring magnet 100 shown in FIG. 1, the ring-shaped powder compact 102 stacked in the third layer (the uppermost layer of the sintered ring magnet 100) should be of a type having the protruding part 103 formed on the upper end surface as depicted in FIG. 20A. More particularly, if every third transferable metal die unit 10 is provided with the upper punch log with the groove g1 having a rectangular cross section (FIG. 8B) or the groove g2 having a trapezoidal cross section (FIG. 8C) formed in the bottom end, a stack of multiple ring-shaped powder compacts 102, or a ring-shaped powder compact rod having a shape like that of the sintered ring-shaped powder compact rod 300 shown in FIG. 2, would be obtained. The upper punch 10 g with the groove g2 having the trapezoidal cross section (FIG. 8C) is preferable to the upper punch 10 g with the groove g1 having the rectangular cross section (FIG. 8B). This is because, if the upper punch 10 g with the groove g2 having a tapered cross-sectional shape as shown in FIG. 8C is used, the ring-shaped powder compact 102 with the protruding part 103 having an upward-narrowing trapezoidal cross section is formed, and the ring-shaped powder compact 102 thus formed is less susceptible to cracking and other damages when released from the upper punch 10 g.

It is impossible to produce a stack of multiple ring-shaped powder compacts by a conventional pressing method in which individual ring-shaped powder compacts are pressed by use of a metal die fixed to a pressing machine. According to the present embodiment, a ring-shaped powder compact rod having the same shape as the sintered ring-shaped powder compact rod 300 of FIG. 2 would be obtained by successively stacking two ring-shaped powder compacts 102 having flat upper and lower end surfaces and one ring-shaped powder compact 102 having a flat lower end surface and the protruding part 103 formed on the upper end surface as shown in FIG. 20B.

If variations in height occur among the individual ring-shaped powder compacts 102 (when the stacked ring-shaped powder compacts 102 become high), undesirable pressure will be exerted on the ring-shaped powder compacts 102 during the stacking process, potentially causing the ring-shaped powder compacts 102 to crush, or the tong-grip lifter 8 a may accidentally release the ring-shaped powder compact 102 in the air, potentially causing breakage of the ring-shaped powder compact 102 as a result of an impact of fall. In the present embodiment, however, the weight of the magnetic powder 11 to be pressed for making the ring-shaped powder compact 102 in each cycle of pressing process is measured to a fixed amount in the magnetic powder weighing process carried out by the powder feeding unit 3, so that the height of each ring-shaped powder compact 102 is made constant and there will not arise such a problem that an undesirable force or an impact force is exerted on the ring-shaped powder compact 102 during the stacking process.

Upon completion of the stacking process, the first holder 10 b, the core 10 d and the lower punch 10 e of the transferable metal die unit 10 are returned onto the palette 10 a by the transfer mechanism 12 and the transferable metal die unit 10 is conveyed to the powder removal/die setup unit 9 where a next process is performed. The powder removal/die setup unit 9 is provided with a powder removal mechanism for removing magnetic powder adhering to the transferable metal die unit 10 and a setup mechanism for setting individual parts of the transferable metal die unit 10 to an initial condition in which the powder feeding unit 3 can feed the magnetic powder 11 again.

The powder removal mechanism has a nozzle for blowing nitrogen gas against the individual parts of the transferable metal die unit 10 and a vacuum mechanism for drawing and collecting the magnetic powder blown off by nitrogen gas. The setup mechanism is a mechanism for lifting the die 10 f placed on the second holder 10 j and moving the die 10 f onto the lower punch 10 e placed on the first holder 10 b upon completion of the stacking process. With the provision of the powder removal mechanism and the setup mechanism, it is possible to smoothly carry out a next cycle of the pressing to staking processes.

The ring-shaped powder compact rod obtained by stacking the ring-shaped powder compacts 102 is transferred to a sintering/heat treatment furnace, in which the ring-shaped powder compact rod is sintered and subjected to heat treatment at a specified temperature. As a result, a sintered ring-shaped powder compact rod like the one shown in FIG. 2 is obtained. The sintered ring-shaped powder compact rod is then divided to obtain a plurality of sintered ring magnets 100 by a procedure already discussed and each sintered ring magnet 100 is mounted on the rotor shaft 200 as shown in FIG. 5.

Second Embodiment

FIG. 22 is a cross-sectional view of a sintered ring-shaped powder compact rod 301 obtained by a method of manufacturing sintered ring magnets according to a second embodiment of the invention. FIG. 23 is a cross-sectional view of the sintered ring magnets obtained by dividing the sintered ring-shaped powder compact rod of FIG. 22. Specifically, a curved inner surface and a curved outer surface of the sintered ring-shaped powder compact rod 301 formed by sintering a plurality of ring-shaped powder compacts 102, 102B are finished by machining, and the sintered ring-shaped powder compact rod 301 is subjected to an anticorrosion surface treatment. Then, a mechanical bending stress is applied to boundary regions of the sintered ring-shaped powder compact rod 301 and, as a result, the sintered ring-shaped powder compact rod 301 breaks at the boundary regions where ring-shaped protruding parts 103 are formed and the sintered ring magnets of the second embodiment are obtained as shown in FIG. 23.

As illustrated in FIG. 22, a ring-shaped powder compact rod from which the sintered ring-shaped powder compact rod 301 of the second embodiment is produced is formed by stacking one ring-shaped powder compact 102B in a lowermost layer and a desired number of ring-shaped powder compacts 102 in upper layers. The ring-shaped powder compact 102B in the lowermost layer is thinner than the other ring-shaped powder compacts 102 and the ring-shaped protruding part 103 is formed on an upper end surface of the ring-shaped powder compact 102B. Although the ring-shaped powder compact 102B in the lowermost layer of the ring-shaped powder compact rod that lies directly on parting powder dusted on a tray would deform due to sintering shrinkage or would not shrink as expected in the sintering operation, the other ring-shaped powder compacts 102 located in the upper layers shrink uniformly without causing sintering deformation. It is therefore possible to obtain the sintered ring-shaped powder compact rod 301 with a high degree of shape accuracy except for the ring-shaped powder compact 102B in the lowermost layer.

Generally, in the manufacture of sintered ring magnets, a substantial proportion of total manufacturing time is devoted to surface finishing operation performed in a later stage. Thus, even if the ring-shaped powder compact 102B which serves as a dummy powder compact is placed in the lowermost layer of the sintered ring-shaped powder compact rod 301 before the sintering operation, the total manufacturing time is shortened due to a reduction in sintering deformation and a consequent reduction in time required for machining. Accordingly, the method of manufacturing the sintered ring magnets of the second embodiment allows for overall cost reduction. Additionally, because a large number of sintered ring magnets 100 can be obtained by using one ring-shaped powder compact 102B serving as a dummy powder compact according to the manufacturing method of the second embodiment, a loss of cost and time needed for the preparation of the dummy ring-shaped powder compact 102B is almost negligible as a whole. It is therefore appreciated that the manufacturing method of the second embodiment contributes to eventual cost savings.

Third Embodiment

FIG. 24 is a cross-sectional view of a sintered ring-shaped powder compact rod 302 obtained by a method of manufacturing sintered ring magnets according to a third embodiment of the invention. Referring to FIG. 24, alumina powder 104 having an average particle size between 1 micrometer and 100 micrometers is dusted to a thickness between 1 micrometer and 100 micrometers in specific boundary regions of ring-shaped powder compacts 102 which are stacked one on top of another for forming the sintered ring-shaped powder compact rod 302. The alumina powder 104 is dusted in half or less of the area of each boundary region.

The ring-shaped powder compacts 102 used for producing the sintered ring-shaped powder compact rod 302 of the third embodiment are made by substantially the same manufacturing method as explained earlier with reference to the first embodiment. For example, to produce the sintered ring-shaped powder compact rod 302 of FIG. 24, three ring-shaped powder compacts 102 are stacked in layers and a small amount of the alumina powder 104 is dusted on an upper end surface of the ring-shaped powder compact 102 in the uppermost layer (third layer). The alumina powder 104 must have an average particle size not exceeding 100 micrometers and the thickness of each layer of the alumina powder 104 should not exceed 100 micrometers, because the ring-shaped powder compacts 102 would not join at the specific boundary regions thereof if the average particle size of the alumina powder 104 or the thickness of the aluminum layer exceeds 100 micrometers. Also, the average particle size of the alumina powder 104 and the thickness of the aluminum layer are made equal to or larger than 1 micrometer, because joint strength at the specific boundary regions would not decrease if the average particle size of the alumina powder 104 or the thickness of the aluminum layer is smaller than 1 micrometer. The sintered ring-shaped powder compact rod 302 is obtained by sintering the ring-shaped powder compacts 102 stacked as shown in FIG. 24 with the alumina powder 104 dusted on the upper end surface of every third ring-shaped powder compact 102 as discussed above. In the sintered ring-shaped powder compact rod 302 of the third embodiment thus produced, the ring-shaped powder compacts 102 are joined with a reduced joint strength at the specific boundary regions.

According to the above-described manufacturing method of the third embodiment, it is possible to easily produce the sintered ring-shaped powder compact rod 302, in which the ring-shaped powder compacts 102 are joined with a reduced joint strength at the specific boundary regions, by a simplified manufacturing process without the need to form protruding parts or recessed parts on the end surfaces of the ring-shaped powder compacts 102 unlike the foregoing embodiments.

It is to be noted that powder of magnesia or other ceramic material may be used instead of the alumina powder 104 used in this embodiment.

Fourth Embodiment

FIG. 25 is a cross-sectional view of a sintered ring-shaped powder compact rod 303 obtained by a method of manufacturing sintered ring magnets according to a fourth embodiment of the invention. Referring to FIG. 25, burrs formed on surfaces of ring-shaped powder compacts 102 during pressing operation and excess magnetic powder which adheres to curved inner surfaces of the ring-shaped powder compacts 102 when each ring-shaped powder compact 102 is drawn out from the core 10d are left unremoved from specific boundary regions 106 of the ring-shaped powder compacts 102.

The ring-shaped powder compacts 102 used for producing the sintered ring-shaped powder compact rod 303 of the fourth embodiment are made by substantially the same manufacturing method as explained earlier with reference to the first embodiment except that the ring-shaped powder compact 102 to be placed immediately below each boundary region 106 is advanced from the die release process to the stacking process, bypassing the powder removal process shown in FIGS. 18A and 18B. Since burrs and/or excess magnetic powder are left at the specific boundary regions 106, upper and lower end surfaces of the ring-shaped powder compacts 102 do not go into tight contact with each other at these boundary regions 106. When the ring-shaped powder compacts 102 thus stacked are sintered, the sintered ring-shaped powder compact rod 303 of the fourth embodiment, in which the ring-shaped powder compacts 102 are joined with a reduced joint strength at the specific boundary regions 106, is obtained.

According to the above-described manufacturing method of the fourth embodiment, it is possible to produce the sintered ring-shaped powder compact rod 303, in which the ring-shaped powder compacts 102 are joined with a reduced joint strength at the specific boundary regions 106, without the need to form protruding parts or recessed parts or dust ceramic powder on the end surfaces of the ring-shaped powder compacts 102 unlike the foregoing embodiments. 

1. A sintered ring magnet comprising a plurality of radially oriented ring-shaped powder compacts which are stacked one on top of another in an axial direction thereof and joined together by sintering, wherein one of a protruding part and a recessed part is formed on at least one of longitudinal end surfaces of said sintered ring magnet.
 2. A method of manufacturing sintered ring magnets comprising the steps of: stacking a plurality of radially oriented ring-shaped powder compacts thereof in an axial direction to produce a ring-shaped powder compact rod; sintering the ring-shaped powder compact rod to produce a sintered ring-shaped powder compact rod in which the ring-shaped powder compacts are joined together as a result of sintering operation; and dividing the sintered ring-shaped powder compact rod; wherein the ring-shaped powder compacts are joined to one another with a reduced joint strength at specific boundary regions than at the other boundary regions when sintered, and said sintered ring magnets are obtained by dividing the sintered ring-shaped powder compact rod at said specific boundary regions having the reduced joint strength.
 3. The method of manufacturing the sintered ring magnets according to claim 2, wherein the ring-shaped powder compacts are stacked on top of a dummy ring-shaped powder compact such that the dummy ring-shaped powder compact is located in a lowermost layer of the ring-shaped powder compact rod, wherein the dummy ring-shaped powder compact is joined to the ring-shaped powder compact stacked immediately above with a reduced joint strength at a boundary region therebetween as a result of the sintering operation, and wherein said sintered ring magnets are obtained by dividing the sintered ring-shaped powder compact rod after the sintering operation at said specific boundary regions having the reduced joint strength.
 4. The method of manufacturing the sintered ring magnets according to claim 2, wherein one of a protruding part and a recessed part is formed on at least one of longitudinal end surfaces of each ring-shaped powder compact facing one of said specific boundary regions, and said sintered ring magnets are obtained by dividing the sintered ring-shaped powder compact rod at each of said specific boundary regions where one of the protruding part and the recessed part is formed after the sintering operation.
 5. The method of manufacturing the sintered ring magnets according to claim 2, wherein ceramic powder having a particle size between 1 micrometer and 100 micrometers is dusted to a thickness between 1 micrometer and 100 micrometers at each of said specific boundary regions, and said sintered ring magnets are obtained by dividing the sintered ring-shaped powder compact rod at said specific boundary regions after the sintering operation.
 6. The method of manufacturing the sintered ring magnets according to claim 2, wherein burrs formed on surfaces of the ring-shaped powder compacts in a pressing process or excess magnetic powder which adheres to the surfaces of the ring-shaped powder compacts in a die release process is left unremoved from said specific boundary regions of the ring-shaped powder compacts, the ring-shaped powder compact rod is produced by using the ring-shaped powder compacts thus prepared, and said sintered ring magnets are obtained by dividing the sintered ring-shaped powder compact rod at said specific boundary regions after the sintering operation.
 7. The method of manufacturing the sintered ring magnets according to claim 2, wherein the sintered ring-shaped powder compact rod is divided at said specific boundary regions having the reduced joint strength after at least one of a curved inner surface and a curved outer surface of the sintered ring-shaped powder compact rod, in which the ring-shaped powder compacts are joined into a single structure by sintering, is finished by machining.
 8. The method of manufacturing the sintered ring magnets according to claim 7, wherein the sintered ring-shaped powder compact rod is divided at said specific boundary regions having the reduced joint strength after the sintered ring-shaped powder compact rod of which at least one of the curved inner surface and the curved outer surface has been machined is subjected to an anticorrosion surface treatment. 